Ellipsometry involves causing a spectroscopic beam of electromagnetic radiation, in a known state of polarization, to interact with a sample, such as, for example, a substrate material surface, at an angle of incidence with respect to a substrate material surface, and often in a plane of incidence. A plane of incidence comprises both a normal plane to a surface being interrogated and the locus of the electromagnetic radiation beam. Changes in the polarization state of the beam that occur as a result of beam interaction with a substrate material surface are indicative of the structure and composition of the substrate material system. Ellipsometry further involves proposing a mathematical model of the ellipsometry system and the substrate material surface being interrogated. Experimental data is obtained from the beam by collecting the beam at a detector after beam incidence at a substrate material surface.
In non-linear optical, or wave mixing processes, outputs are produced at sum, difference or harmonic frequencies of the input beam(s), otherwise equivalently referred to as signal(s). Second order non-linear optics, or three wave mixing involves combining two input signals to produce one output signal at one of the combined frequencies. The use of second order nonlinear optical surface spectroscopy to examine physical properties of a material surface is known. However, practical constraints on such known methods have impeded progress on the material evaluation and characterization below the surface of a material under inspection.
Composite materials such as fiberglass, Kevlar, and carbon fiber are increasingly being used as structural components in aircraft because of their high strength-to-weight ratios, improved performance, reduced corrosion, etc. compared with other known structural materials. However, composites can be weakened by various defects and stress during their life cycle. Routine maintenance of composites requires complicated inspection and repair techniques.
One particular aspect of desired surface analysis and interrogation is the need to characterize the alignment of surface molecules occurring at a substrate material surface and/or any non-native species that may be present on the surface, (e.g. contaminants in the material itself, or contaminants associated with material processing and manufacturing, etc.). Non-invasive, on-site surface analysis has proven difficult, even under optimum environmental conditions, while often constraining implementation of the known diagnostics to a manufacturing environment. Once a material is processed into a finished product, use of known surface interrogation techniques, systems and apparatuses for monitoring the condition of surfaces in the field to assess surface conditions, such as, for example, material fatigue, stress, etc., has not been possible, or has yielded unreliable and non-repeatable data.